Previous investigations into the effect of endurance (aerobic) exercise training on the regulation of blood pressure during an orthostatic challenge suggests that orthostatic tolerance is reduced because of an attenuated baroreflex. Endurance exercise training is known to produce an increase in total blood volume, primarily plasma volume and a parasympathetically dominated autonomic nervous system (ANS). Hence, it is hypothesized that the exercise induced alteration in the balance between the parasympatheti and sympathetic effectors of the ANS are responsible for the attenuated baroreflex. This hypothesis will be examined in humans using two experimental approaches. The first approach uses a cross-sectional comparison of the cardiovascular responses to lower body negative pressure (LBNP) between endurance exercise trained, weight trained and untrained subjects. In which: (a) the effect of the parasympathetic arm of the ANS has been modified by partial or full blockade with atropine, or (b) the effect of Beta1 receptor activation has been modified by partial or full blockade using metoprolol. The second approach uses a longitudinal investigation of the effects of 10 weeks of endurance exercise training. Pre-post training comparisons of the cardiovascular responses to LBNP across three distinct fitness groups (high, middle and low) using both full and partial blockade of the parasympathetic and Beta1 sympathetic arms of the ANS will discern the effects of endurance training from those of initial training status. All comparisons in both the cross-sectional and longitudinal approaches will be evaluated with respect to absolute total blood volume, plasma volume, volume of blood translocated during LBNP and the electromyographic activity of the lower limb during LBNP. It is expected that the data obtained in this investigation will identify those physiological effects of exercise training that produce an alteration in blood pressure regulation during orthostasis. Furthermore the data will form a basis upon which the observed increased incidence of orthostatic intolerance in the elderly can be investigated.